Steroid hormones have been demonstrated to induce in tissue culture the production of mouse mammary tumor viral (MMTV) RNA, proteins, and particles 10-fold compared with constitutive levels. However, previous data of increased viral RNA levels did not distinguish between an increased rate of viral-specific RNA synthesis and a slower rate of viral RNA degradation. According to the recently developed assay of Coffin et al. (1974) for measuring rates of viral RNA synthesis, short-term labeling experiments of a mouse mammary tumor cell line indicate that the glucocorticoid hormone dexamethasone stimulates a 3-fold increase in the synthesis of MMTV-specific RNA within 10 min after the addition of hormone and that stimulation of RNA synthesis reaches 5- to 10-fold within 30 to 60 min, while the synthesis of Moloney leukemia virus-specific RNA in the same cell is unaffected by steroids. The decay rates of pulse-labeled and accumulated MMTV RNA in the presence or absence of dexamethasone show this RNA to have a half-life of greater than 8 h. In addition, hormone-stimulated MMTV RNA appears to have an increased rate of decay compared to basal MMTV RNA, thus ruling out an increased stability of MMTV RNA in the presence of steroid hormones as the basis for increased RNA levels. Thus, the magnitude, rapidity, and specificity of hormone action on MMTV RNA synthesis indicate a primary effect upon transcription.